The present embodiments relate to an electronic control circuit which is particularly, though not exclusively, suited to the ballasting of low and high pressure sodium, mercury arc and metal halide discharge lamps (high intensity discharge lamps or HID lamps). Typically, such systems are used for highway lighting, architectural floodlighting, warehouse, retail display lighting or industrial lighting.
FIG. 4 of U.S. Pat. No. 6,384,544 shows a circuit topology with two independent buck converters operable in two modes. A frequency control circuit controls an oscillator to output a square wave. The output of the oscillator is split, one half being passed through an inverter, to create two complementary outputs in anti-phase with each other. These anti-phase outputs are connected to the inputs of two dual input AND gates, the other inputs of the two AND gates being connected to the output of a voltage comparator. The outputs of the two AND gates are connected to a MOS gate driver IC, which drives a pair of MOSFETs via gate drive resistors. Each MOSFET is serially connected to a fast recovery diode. The respective MOSFET to diode connection nodes A and AA are further connected to separate inductors, which are connected to each other at node B, which is also connected to capacitor and lamp. The capacitor is connected to node C. The lamp is returned to node C via the primary winding of current transformer. Node C provides a current return path for capacitor and lamp via capacitors to the +HT and 0V rails.
In the first mode of operation, the frequency control circuit sets the oscillator frequency to typically several tens of kilohertz. The output of the voltage comparator (node F) is a logic 1 so the anti-phase complementary outputs from the oscillator and the inverter are “passed” by the AND gates, driving the inputs of the MOS gate driver IC, which in turn drives the gates of the MOSFETs. The alternate switching of the MOSFETs alternately connects node A to the +HT rail and node AA to the 0V rail so that the LC resonant circuit comprising the inductors and a capacitor is stimulated alternately via node A and one of the inductors and via node AA and the other of the inductors at the fundamental resonant frequency of the resonant LC components or a harmonic thereof. Voltage multiplication occurs at node B owing to the Q-factor of the resonant components. The resonant components are designed with sufficient Q-factor to provide a voltage capable of ionising the gas filling the arc tube of lamp, thus initiating an arc at the lamp electrodes.
This arc is sustained by current flowing via the primary winding of a current transformer and node C to the capacitors, which allow the current to return to the +HT and 0V rails. The arc impedance is sufficiently low to divert most of the current flowing in the inductors away from the capacitor and via the lamp. The circuit operates in this first mode until the lamp electrodes are sufficiently heated to establish thermionic emission. The circuit is then switched to the second of the two discrete modes of operation.
In the second mode of operation, the frequency control circuit sets the oscillator to a second, lower frequency, typically though not exclusively several tens or hundreds of hertz. Since thermionic emission is already established in the lamp by the heating of the electrodes in the first mode of operation, the voltage available at the lamp terminals in this second (non-resonant) mode of operation is sufficient to maintain the arc at the lamp electrodes. The output of the voltage comparator (node F) is a logic 1 so the anti-phase complementary outputs from the oscillator and the inverter are “passed” by the AND gates, driving the inputs of the MOS gate driver IC, which in turn drives the gates of the MOSFETs. The alternate switching of the two MOSFETs on and off in opposition alternately connects node A to the +HT rail and node AA to the 0V rail. In one half cycle of the oscillator, one of the MOSFETs conducts current from the +HT rail to the lamp via node A, one of the inductors and node B; and in the opposing half cycle of the oscillator, the other of the MOSFETs conducts current from the 0V rail to the lamp via node AA, another of the inductors and node B.
Lamp current is transformed by the turns ratio of the current sensing transformer, rectified by a rectifier and converted to a positive voltage proportional to lamp current across a resistor. This voltage appears at node D and is referenced to the 0V rail. Node D is connected to the inverting input of a voltage comparator. The voltage at node D is compared with a voltage set by a potential divider (two resistors), the mid point of which (node E) is connected to the non-inverting input of the voltage comparator. Should the lamp current proportional voltage at node D exceed the voltage set by the potential divider at node E, the output (node F) of the voltage comparator is switched to a logic 0 state. Since node F is connected to the inputs of the AND gates 3 and 4, both outputs from the AND gates are then forced to a logic 0 level irrespective of the logic states of the other inputs to the AND gates set by the outputs of the oscillator and the inverter. Whichever MOSFET was conducting and sourcing current into the lamp circuit is switched to a non-conducting state and reactive current flowing in the associated inductor is circulated via the fast recovery diode. When the current value decays sufficiently to reduce the lamp current proportional voltage at node D to a voltage below that set at node E, the voltage comparator output node F returns to a logic 1 state allowing the AND gates to “pass” the relevant logic states set on their other inputs and thus to switch the relevant MOSFET to a conducting state.
Typically, the lamp current in the first mode of operation is insufficiently high to trigger operation of this current limit circuit. Since the oscillator frequency in the second mode of operation is substantially lower than in the first mode of operation, the inductors pass much higher currents as their impedance at low frequency is much lower than their impedance at high frequency. Due to the operation of the current limiting circuit, the lamp current waveform in the second mode approximates a square wave with some high frequency ripple due to the operation of the current limiting circuit superimposed on the top and bottom extremities of the waveform.
The ripple current at the lamp caused by the switching of the buck converters in the second, continuous mode of operation may not meet requirements of recent proposed international standards. The standards limit the amplitude of high frequency ripple on the lamp waveform so as to reduce the possibility of acoustic arc resonance. Recent environmental concerns have also reduced the use of radioactive elements in discharge lamps. Lamps with no or reduced radioactive krypton may be more difficult to strike. In the circuit of U.S. Pat. No. 6,384,544, a higher striking voltage may be needed.